More riot-web test deflakification
More riot-web test deflakification

Two changes:

1. wait longer for /sync to arrive in the loading tests, via an
   `expectAndAwaitSync` method.

2. https://github.com/matrix-org/matrix-react-sdk/pull/773 made it possible for
   MatrixChat to not show its syncing spinner despite `loading` being
   false. Update `awaitSyncingSpinner` accordingly, so that it doesn't fail
   when it happens to check MatrixChat at just taht moment.

martindale/vector,vector-im/vector-web,vector-im/vector-web,vector-im/riot-web,vector-im/riot-web,vector-im/riot-web,vector-im/riot-web,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,vector-im/vector-web,martindale/vector,vector-im/vector-web